Semiconductor crystal producing method

ABSTRACT

In a separation layer removing process α, temperature in a reaction chamber (heat treatment temperature T X ) is raised to about 1000° C. and a separation layer A is evaporated through thermal decomposition, to thereby separate about 10 μm in thickness of protection layer B from a base substrate side (a sapphire substrate  101  comprising a buffer layer  102 ). Because decomposition temperature of the separation layer A is higher than growth temperature of the protection layer B (about 650° C.) and lower than growth temperature of the semiconductor crystal C (about 1000° C.), the separation layer A vanishes (evaporates) by thermal decomposition, which generates this separation process. Accordingly, a semiconductor crystal having a cross sectional structure shown in FIG. 2B is obtained. By employing the protection layer B which is independent from the base substrate side as another crystal growth substrate, dislocations and cracks may not be generated by stress owing to difference of lattice constants or difference of thermal expansion coefficients, and a semiconductor crystal layer C (GaN single crystal) of high quality can be obtained.

TECHNICAL FIELD

[0001] The present invention relates to a method for growing asemiconductor crystal comprising group III nitride compoundsemiconductor on a base substrate, to thereby obtain a semiconductorcrystal which has excellent quality and is free-standing from the basesubstrate.

[0002] And the present invention can be applied to a production of acrystal growth substrate for various semiconductor devices representedby e.g., LEDs.

BACKGROUND ART

[0003] A conventional technique in which a semiconductor crystalconsisting of a group III nitride compound semiconductor is grown on abase substrate and a free-standing semiconductor crystal is obtainedindependently from the base substrate is generally known as, forexample, wet etching disclosed in a Japanese Patent ApplicationLaid-Open (kokai) No. 7-202265 entitled “Manufacture of Group IIINitride Semiconductor” and a method comprising steps of growing a thickGaN (semiconductor crystal to be obtained) on a sapphire substrate byusing HVPE or other process and removing the sapphire substrate byirradiating laser or polishing.

[0004] In a conventional method, difference of thermal expansioncoefficients and difference of lattice constants between the basesubstrate (e.g., sapphire and so on) and a group III nitride compoundsemiconductor result in applying stress to the objective single crystal(e.g., GaN) when the semiconductor is cooled to ambient temperatureafter growing process, thereby generating a number of dislocations andcracks in the objective single crystal.

[0005] When the objective single crystal is grown to a certainthickness, cracks are generated in the objective single crystal evenduring the growth process. That easily causes problems such that smallpieces are partially peeled off.

DISCLOSURE OF THE INVENTION

[0006] The present invention has been accomplished in order to overcomethe aforementioned drawbacks. Thus, an object of the present inventionis to produce a semiconductor of high quality crystal which isindependent from the base substrate.

[0007] The following means may be useful to overcome the above-describeddrawbacks.

[0008] That is, the first aspect of the present invention provides amethod for producing a semiconductor crystal having excellent qualityand independent from a base substrate on which a semiconductor crystalcomprising a group III nitride compound semiconductor is grown,comprising: single crystal growth process c in which an objectivesemiconductor crystal layer C comprising a group III nitride compoundsemiconductor is deposited; and separation layer removing process α inwhich a separation layer A which comprises a group III nitride compoundsemiconductor and is formed between a protection layer B comprising agroup III nitride compound semiconductor and having crystal growth fronttoward the semiconductor crystal layer C and the base substrate isevaporated through thermal decomposition, wherein the separation layerremoving process α is carried out before the single crystal growthprocess c.

[0009] That is, for example, the separation layer A comprising materialssuch as Ga_(0.5)In_(0.5)N and the protection layer B comprisingmaterials such as Ga_(0.8)In_(0.2)N may be formed between the basesubstrate comprising materials such as sapphire and the objectivesemiconductor crystal layer C comprising materials such as GaN film.

[0010] By applying such structure, the temperature at which theseparation layer A evaporates through thermal decomposition can bearranged lower than the temperature at which the protection layer Bevaporates through thermal decomposition, and that makes it possible todecompose only the separation layer A through thermal decomposition byraising the temperature.

[0011] Because only the separation layer A can be removed throughthermal decomposition before forming the semiconductor crystal layer C,the semiconductor crystal layer C may be grown without being influencedby stress generated from the base substrate. That is, the semiconductorcrystal layer C which is formed on the free-standing protection layer Bmay not receive worse affection, i.e., stress owing to difference oflattice constants and difference of thermal expansion coefficientsgenerated from the base substrate.

[0012] Accordingly, because the base substrate can be separated from theprotection layer B before forming the thick semiconductor crystal layerC (e.g., GaN film), no dislocations or cracks owing to difference oflattice constants and difference of thermal expansion coefficientsoccur, and a semiconductor crystal layer C of high quality can beobtained. The thus-obtained thick semiconductor crystal layer C (e.g.,GaN film) can be employed, for example, as a crystal growth substrate ina semiconductor device. So industrial utility value of the semiconductorcrystal layer C is large.

[0013] And because the above producing method can be carried out only ina crystal growth process, processes such as laser irradiation andpolishing become unnecessary. As a result, the base substrate remainswithout being polished or cracked, which makes it possible to reuse thebase substrate.

[0014] The second aspect of the present invention is drawn to the methodaccording to the first aspect, wherein the group III nitride compoundsemiconductor is made of Al_(1-x-y)Ga_(y)In_(x)N (0≦x≦1, 0≦y≦1,0≦1-x-y≦1).

[0015] Here a group III nitride compound semiconductor generallyincludes a binary, ternary, or quaternary semiconductor represented by aformula Al_(1-x-y)Ga_(y)In_(x)N (0≦x≦1, 0≦y≦1, 0≦1-x-y≦1) and having anarbitrary composition ratio. And a group III nitride compoundsemiconductor in the present specification further includes asemiconductor doped with p-type or n-type impurity.

[0016] Alternatively, a semiconductor whose portion of the group IIIelements (Al, Ga, In) may be replaced with boron (B) or thallium (Tl),and a portion of nitrogen (N) may be replaced with phosphorous (P),arsenic (As), antimony (Sb), or bismuth (Bi) is also included in a groupIII nitride compound semiconductor of the present specification.

[0017] Examples of the p-type dopant which can be added includemagnesium (Mg) and calcium (Ca).

[0018] Examples of the n-type dopant which can be added include silicon(Si), sulfur (S), selenium (Se), tellurium (Te), and germanium (Ge).

[0019] These dopants may be used in combination of two or more species,and a p-type dopant and an n-type dopant may be added simultaneously.

[0020] In the present specification, group III nitride compoundsemiconductor may simply be described as a group III nitride compound.

[0021] The third aspect of the present invention is drawn to the methodaccording to the first or second aspect, wherein the base substrate isformed by using sapphire, spinel, manganese oxide, lithium gallium oxide(LiGaO₂), molybdenum sulfide (MoS), silicon (Si), carbon silicide (SiC),AlN, GaAs, InP, GaP, MgO, ZnO, or MgAl₂O₄.

[0022] Any material which can endure at a temperature in the heattreatment while carrying out the separation layer removing process α maybe employed to form the base substrate. In short, a well-known orarbitrary crystal growth substrate which is useful for crystal growth ofa group III nitride compound semiconductor may be used as materials toform the base substrate.

[0023] The fourth aspect of the present invention is drawn to the methodaccording to any one of the first to third aspects, wherein therelationship among the heat treatment temperature T_(X) in theseparation layer removing process α, the crystal growth temperatureT_(C) of the semiconductor crystal layer C, and the crystal growthtemperature T_(B) of the protection layer B is T_(B)<T_(X)≦T_(C).

[0024] The decomposition temperature (T_(X)) of the separation layer Acan be arranged between the growth temperature (T_(B)) of the protectionlayer B and the growth temperature (T_(C)) of the semiconductor crystallayer C by controlling compositions of the separation layer A and theprotection layer B. And, by arranging the decomposition temperatureT_(X) between T_(B) and T_(C), the separation layer A can be vanishedthrough thermal decomposition occurred in a heating process by the timethe semiconductor crystal layer C is formed. As a result, undesirableaffection such as stress from the base substrate can be spontaneouslyprevented and the semiconductor crystal layer C can be grown.

[0025] The fifth aspect of the present invention is drawn to the methodaccording to any one of the first to fourth aspects, wherein theprotection layer B is formed to have indium (In) composition ratiosmaller than the indium (In) composition ratio of the separation layerA. That is, in the semiconductor crystal having compositions ofIn_(x)Ga_(1-x)N, indium (In) composition ratio x1 of the protectionlayer B may preferably be about 0.2 and indium (In) composition ratio x2of the separation layer A may preferably be about 0.5 (>x1). By applyingsuch indium (In) composition ratios x1 and x2, for example, only theseparation layer A can be sufficiently removed while the protectionlayer B with excellent quality can stably remain in the separationremoving process α.

[0026] The sixth aspect of the present invention is drawn to the methodaccording to any one of the first to fifth aspects, wherein thesemiconductor crystal layer C is made of gallium nitride (GaN) at atemperature of 900° C. to 1100° C. through crystal growth.

[0027] As a composition of the semiconductor crystal layer C, galliumnitride (GaN) is the most optimum and very useful to form a crystalgrowth substrate of a semiconductor and is considered to have thehighest industrial utility. And by forming the gallium nitride (GaN)around 1000° C. through crystal growth, a single crystal having thehighest quality can be obtained.

[0028] AlGaN and AlGaInN also have high industrial utility. SO they maybe alternatively used as compositions of the semiconductor crystal layerC.

[0029] The seventh aspect of the present invention is drawn to themethod according to any one of the first to sixth aspects, wherein theprotection layer B is made of Ga_(1-x)In_(x)N (0.0≦x≦0.8) throughcrystal growth. More preferably, indium (In) composition ratio x of theprotection layer B may be in a range of 0.05 to 0.5.

[0030] Further preferably, although it depends on indium (In)composition ratio of the separation layer A and the temperature of heattreatment in the separation layer removing process α, indium (In)composition ratio x of the protection layer B may be in a range of 0.1to 0.3. By arranging the indium (In) composition ratio x in such arange, for example, the protection layer B having excellent quality canremain stably in the separation layer removing process α.

[0031] Alternatively, it is also possible to form the protection layer Bby using GaN.

[0032] Thickness of the protection layer B may preferably be in a rangefrom 0.1 μm to 100 μm, and more preferably in a range from 5 μm to 20μm. When thickness of the protection layer B is too large, it takesunnecessarily longer time for crystal growth, which is not desirable.When thickness of the protection layer B is too small, the protectionlayer B tends to be ruptured or damaged while handling the protectionlayer B in the substrate taking out process, which is not desirable.

[0033] The eighth aspect of the present invention is drawn to the methodaccording to any one of the first to seventh aspects, wherein theseparation layer A is formed through crystal growth by usingGa_(1-x)In_(x)N (0.05≦x≦1.0). More preferably, indium (In) compositionratio x of the separation layer A is in a range from 0.3 to 0.7.

[0034] Further preferably, although it depends on indium (In)composition ratio of the protection layer B and the temperature of heattreatment in the separation layer removing process α, indium (In)composition ratio x of the separation layer A may be in a range of 0.4to 0.6. By arranging the indium (In) composition ratio x in such range,for example, the separation layer A can be sufficiently removed in theseparation layer removing process α.

[0035] Thickness of the separation layer A may preferably be in a rangefrom 0.1 μm to 100 μm, and more preferably in a range from 5 μm to 20μm. When thickness of the separation layer A is too large, it takesunnecessarily longer time for crystal growth, which is not desirable.When thickness of the separation layer A is too small, it tends tohappen that the base substrate (e.g.: sapphire) and the protection layerB cannot be separated securely, which is not desirable.

[0036] The ninth aspect of the present invention is drawn to the methodaccording to any one of the first to eighth aspects, wherein thetemperature T_(X) of heat treatment carried out in the separation layerremoving process α is in a range from 700° C. to 1100° C.

[0037] Although it depends on composition of each semiconductor layersA, B and C, the optimum value of the temperature T_(X) may mostpreferably be the same or around 1000° C. in order to reduce the timefor heat treatment and to maintain crystallinity of the protection layerB.

[0038] The tenth aspect of the present invention is drawn to the methodaccording to any one of the first to ninth aspects, further comprising aprotection layer removing process β in which the protection layer B isevaporated by heat treatment is carried out after the single crystalgrowth process c.

[0039] For example, by raising the temperature in a reaction chamber tothe decomposition temperature of the protection layer B (e.g.: around1100° C. to 1200° C.) after forming the semiconductor crystal layer Ccomprising such as GaN by crystal growth, the protection layer B canalso be vanished and only the semiconductor crystal layer C remains. Asa result, a free-standing semiconductor single crystal having singlelayer structure can be obtained.

[0040] Here, the protection layer B may not be necessarily removed aftercarrying out the single crystal growth process c. Whether the protectionfilm removing process β is carried out in the present invention or notmay be determined comprehensively by fully considering each condition ofthe semiconductor device such as structure, function, performance, use,unit price, and so on.

[0041] The eleventh aspect of the present invention is drawn to themethod according to the tenth aspect, wherein the heat treatmenttemperature in the protection layer removing process β is in a rangefrom 900° C. to 1200° C. Although it depends on compositions of theprotection layer B and the semiconductor crystal layer C, the optimumvalue of the heat treatment temperature enables to suppress damages tothe semiconductor crystal layer C and effectively vanish the protectionlayer B.

[0042] When the heat treatment temperature is too high, damages towardthe semiconductor crystal layer C become larger and a semiconductorcrystal having excellent crystallinity cannot be obtained. When heattreatment temperature is too low, it becomes difficult to remove theprotection layer B securely or it takes much longer time to remove theprotection layer B, which is not desirable.

[0043] For example, when the semiconductor crystal layer C is made ofGaN and indium (In) composition ratio x of the protection layer B isaround 0.2, the heat treatment temperature in the protection layerremoving process β may preferably around 1150° C. Accordingly, theprotection layer B can be sufficiently removed while crystallinity ofthe semiconductor crystal layer C is maintained sufficiently excellent.

[0044] The twelfth aspect of the present invention is drawn to themethod according to any one of the first to eleventh aspects, furthercomprising a cap layer depositing process in which a cap layer, whichprotects the protection layer B from thermal decomposition in theseparation layer removing process α, is deposited on the protectionlayer B before the separation layer removing process α.

[0045] The compositions, the growth temperature and the thickness of thecap layer may preferably be determined so that the cap layer vanishessuitably by thermal decomposition by the time when the semiconductorcrystal layer C starts to grow.

[0046] Accordingly, by depositing and vanishing the cap layer,crystallinity of the surface of the protection film B (crystal growthfront of the semiconductor crystal layer C) can be maintainedsufficiently excellent at a high temperature during heat treatment andso on. As a result, the semiconductor crystal layer C can become asingle crystal having excellent crystallinity more securely.

[0047] Also, it is effective to form the cap layer to have approximatelythe same composition as that of the semiconductor crystal layer C. Thatenables to start and carry on crystal growth of the semiconductorcrystal layer C having excellent quality even when all of the cap layercannot be removed but a portion of the cap layer remains on theprotection layer B.

[0048] The conventional arts relating to such a cap layer are disclosedin, for example, a Japanese patent application Laid-open No. H11-68159:Group III Nitride Compound Semiconductor Device. In that prior artdocument, basic principle with respect to the cap layer of the presentinvention is explained more generally.

[0049] The thirteenth aspect of the present invention is drawn to themethod according to the twelfth aspect, wherein the cap layer comprisesa group III nitride compound semiconductor having approximately the samecomposition as that of the semiconductor crystal layer C is grownthrough crystal growth at a growth temperature lower than the crystalgrowth temperature T_(C) of the semiconductor crystal layer C.

[0050] The fourteenth aspect of the present invention is drawn to themethod according to the twelfth or thirteenth aspect, wherein a galliumnitride (GaN) is grown through crystal growth as the cap layer at agrowth temperature from 800° C. to 1000° C.

[0051] By applying those conditions, for example, the cap layerdepositing process as described in the twelfth aspect can be smoothlycarried out.

[0052] The fifteenth aspect of the present invention is drawn to themethod according to any one of the first to fourteenth aspects, whereinthe crystal growth of all or at least the latter part of the singlecrystal growth process c is carried out by a second crystal growth(e.g.: HVPE) having a growth rate faster than that of the crystal growth(e.g.: MOVPE) which had been carried out by that time.

[0053] For example, by the time when the semiconductor crystal layer Cis formed to have a certain thickness (e.g.: about 10 μm to 50 μm), acrystal growth method which can grow a single crystal of excellentquality (e.g.: MOVPE) is employed and then a crystal growth method whichcan effectively grow thick semiconductor crystal layer (e.g.: HVPE) isemployed. That enables to supply a semiconductor crystal havingexcellent quality and optimum thickness (e.g.: about 100 μm to 200 μm)in a shorter time.

[0054] The sixteenth aspect of the present invention is drawn to themethod according to any one of the first to fifteenth aspects, whereinthe crystal growth temperature T_(A) of the separation layer A and thecrystal growth temperature T_(B) of the protection layer B are arrangedfrom 400° C. to 900° C.

[0055] By applying that range of crystal growth temperature, theprotection layer B which provides a growth front to the objectivesemiconductor crystal layer C can have excellent crystallinity. Eachcrystal growth temperature of the semiconductor layers A and B maypreferably be about 450° C. to 700° C.

[0056] More preferably, the growth temperature of the separation layer Amay be about 500° C. to 700° C. when the indium composition ratio x ofthe separation layer A is about 0.4 to 0.6. And the growth temperatureof the separation layer B may preferably be about 600° C. to 800° C.when the indium composition ratio x of the separation layer B is about0.1 to 0.3. By applying such ranges of growth temperature, a singlecrystal film having an objective indium composition ratio can be formedto have high quality and at the same time the protection layer havingexcellent quality can be remained stably in the separation layerremoving process α.

[0057] The seventeenth aspect of the present invention is drawn to themethod according to any one of the first to sixteenth aspects, whereinthe relationship between the crystal growth temperature T_(A) of theseparation layer A and the crystal growth temperature T_(B) of theseparation layer B is T_(A)≦T_(B).

[0058] That is, when the crystal growth temperature T_(A) of theseparation layer A is about 500° C. and the crystal growth temperatureT_(B) of the separation layer B is about 650° C., for example, a singlecrystal (semiconductor crystal layer C) having excellent crystallinitycan often be obtained. Moreover, the crystal growth temperatures T_(A)and T_(B) can be arranged closer with each other.

[0059] Accordingly, the protection layer B can be stable more than theseparation layer A during carrying out heat treatment in the separationlayer removing process.

[0060] Through employment of the aforementioned aspects of the presentinvention, the aforementioned drawbacks can be overcome effectively andrationally.

BRIEF DESCRIPTION OF THE DRAWINGS

[0061]FIG. 1 is a graph showing a temperature in a reaction chamberbefore carrying out a single-crystal growth process c according to thefirst embodiment of the present invention.

[0062]FIGS. 2A-2D are sectional views of a group III nitride compoundsemiconductor according to the first embodiment of the presentinvention.

[0063]FIGS. 3A-3D are schematic views of a group III nitride compoundsemiconductor according to the second embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0064] Embodiments of the present invention will next be described basedon concrete examples. The scope of the present invention, however, isnot limited to the embodiment described below.

[0065] (First Embodiment)

[0066] In the first embodiment of the present invention, MOVPE iscarried out as the former part of a process for growing a single crystalin which an objective semiconductor crystal layer C made of group IIInitride compound semiconductor (GaN) is deposited (a former process inwhich the semiconductor crystal layer C grows to have thickness of about30 μm), and HVPE is carried out as the latter part of the process (thelatter part of the process in which the semiconductor crystal layer Cgrows to have thickness of about 150 μm).

[0067]FIG. 1 is a graph showing variation of temperature in a reactionchamber before carrying out a single crystal growth process c (theformer part of the process described above) in the first embodiment ofthe present invention. In FIG. 1, “a” represents a separation layerdeposition process in which a separation layer A made of a first groupIII nitride compound semiconductor (Ga_(0.50)In_(0.50)N) through crystalgrowth (MOVPE), “b” represents a protection layer deposition process inwhich a protection layer B made of a second group III nitride compoundsemiconductor (Ga_(0.80)In_(0.20)N) through crystal growth (MOVPE), and“c” represents a single-crystal growth process in which a third groupIII nitride compound semiconductor, or an objective semiconductorcrystal layer C is deposited through crystal growth (MOVPE).

[0068] And “α” in the graph represents a separation layer removingprocess in which the separation layer A is evaporated to vanish in aheat treatment (increasing temperature).

[0069]FIGS. 2A-2D are sectional views showing the structure of a groupIII nitride compound semiconductor before and after each process carriedout in the first embodiment.

[0070] In the following Example, a method for producing a GaN singlecrystal (semiconductor crystal layer C) according to a first embodimentof the present invention and FIGS. 1 and 2A-2D will be described.

[0071] 1. Cleaning Process

[0072] First, a reaction chamber is exhausted, and a susceptor on whicha sapphire substrate 101 is placed is heated to be about 1000° C., tothereby clean the crystal growth front of the sapphire substrate 101.

[0073] 2. MOVPE Method (Before Separation)

[0074] (1) Buffer Layer Forming Process

[0075] Next, a buffer layer 102 made of AlN and having a thickness ofabout 500Å is formed on the crystal growth front of a sapphire substrate101. Crystal growth temperature at this time is about 420° C.

[0076] (2) Separation Layer Depositing Process a

[0077] Next, a separation layer A made of a first group III nitridecompound semiconductor (Ga_(0.50)In_(0.50)N) and having a thickness ofabout 10 μm is formed. Crystal growth temperature T_(A) at this time isabout 500° C.

[0078] (3) Protection Layer Depositing Process b

[0079] Next, a protection layer B made of a second group III nitridecompound semiconductor (Ga_(0.80)In_(0.20)N) and having a thickness ofabout 10 μm is formed. Crystal growth temperature T_(A) at this time isabout 650° C.

[0080] Accordingly, a semiconductor crystal having a structure shown inFIG. 2A was obtained through the method described above.

[0081] 3. Separation Layer Removing Process α

[0082] Next, a reaction chamber is exhausted and heated until thetemperature of the chamber (heat treatment temperature T_(X)) becomesabout 1000° C., and the separation layer A described above isevaporated, to thereby separate about 10 μm in thickness of theprotection layer B from the sapphire substrate 101 on which the bufferlayer 102 is formed.

[0083] Because decomposition temperature of the separation layer A issmaller than temperature for forming the protection layer B (about 650°C.) and temperature for forming the semiconductor crystal layer C (about1000° C.), the separation layer A is vanished (evaporated) and theprotection layer B is separated from the base substrate (sapphiresubstrate 101).

[0084] Accordingly, a semiconductor crystal having a structure shown inFIG. 2B was obtained through the method described above.

[0085] 4. Base Substrate Taking Out Process

[0086] Next, by cooling the reaction chamber to the ambient temperature,the sapphire substrate 101 (base substrate) on which the buffer layer102 is formed and about 10 μm in thickness of protection layer B arecooled, to thereafter take out the sapphire substrate 101 from thereaction chamber.

[0087] 5. Single Crystal Growth Process

[0088] (1) Evacuating and Heating Process

[0089] Next, the reaction chamber is evacuated and heated until itstemperature (crystal growth temperature T_(C) of the semiconductorcrystal layer C) becomes about 1000° C.

[0090] (2) Single Crystalline Growth Process c (Former Part)

[0091] Next, through MOVPE, a part of an objective semiconductor crystallayer C made of GaN (about 30 μm in thickness) is deposited.

[0092] (3) Single Crystalline Growth Process c (Latter Part)

[0093] Then, through HVPE (the second crystal growth process), crystalgrowth is carried out until the above-described semiconductor crystallayer C grows to have a thickness about 150 μm on about 30 μm inthickness of GaN single crystal (a part of the semiconductor crystallayer C) which functions as a substrate. At this time, crystal growthtemperature may be around 1000° C.

[0094] Accordingly, a semiconductor crystal having a structure shown inFIG. 2C was obtained through the method described above.

[0095] 6. Protection Layer Removing Process β

[0096] Then, the protection layer B is heated to its decompositiontemperature (about 1150° C.), and it is vanished through thermaldecomposition (vaporization). As a result, only the semiconductorcrystal layer C (GaN layer) remains, resulting in obtaining afree-standing GaN substrate.

[0097] Accordingly, a semiconductor crystal having a structure shown inFIG. 2D was obtained through the method described above.

[0098] In the present invention, the present process β may beabbreviated.

[0099] Accordingly, the base substrate can be separated from theprotection layer B before forming a thick semiconductor crystal layer C(GaN film), and a semiconductor crystal layer C having a sufficientthickness and excellent quality, in which no dislocations nor cracksowing to difference of lattice constants or thermal expansioncoefficients with the base substrate (sapphire substrate 101) aregenerated, is obtained through the method described above.

[0100] (Second Embodiment)

[0101]FIGS. 3A-3D are sectional views showing the structure of a groupIII nitride compound semiconductor before and after each process carriedout in the second embodiment of the present invention.

[0102] This second embodiment is basically carried out according to themethod of the first embodiment. Further, in this embodiment, after theprotection layer depositing process b, the following step is carriedout: a GaN layer C′ (cap layer C′) is grown to be a thickness of about20 μm in a temperature of about 900° C. on the Ga_(0.80)In_(0.20)Nprotection layer B having thickness of about 10 μm; and the temperatureis then raised to be about 1000° C. to evaporate the separation layer Aarranged in a middle layer, to thereby separate the base substrate sidewhich comprises the sapphire substrate 101 and the AlN buffer layer 102and the new crystal growth substrate side which comprises the GaN layerC′ and the protection layer B.

[0103] By applying the above-described method, for example, the caplayer C′ (GaN layer C′) which functions as a protection film enables tomaintain crystallinity of the protection layer B, which tends to bedamaged by thermal decomposition and so on, comparatively stable arounda temperature of 900° C. to 1000° C. As a result it is possible or easyto favorably start the single crystalline growth process c in which asemiconductor crystal layer C is formed through crystal.

[0104] The compositions, the growth temperature and the thickness of thecap layer C′ may preferably be determined so that the cap layer vanishessuitably by the time when the semiconductor crystal layer C starts togrow.

[0105] Accordingly, by depositing and vanishing the cap layer C′,crystallinity of the surface of the protection film B (crystal growthfront of the semiconductor crystal layer C) can be maintainedsufficiently excellent at a high temperature during heat treatment andso on. As a result, the semiconductor crystal layer C can become asingle crystal with high quality more securely.

[0106] For example, as explained above, generally it is effective toform the cap layer C′ to have approximately the same composition as thatof the semiconductor crystal layer C. That enables to start and carry oncrystal growth of the semiconductor crystal layer C having an excellentquality in case that all the cap layer C′ cannot be removed but aportion of the cap layer C′ remains on the protection layer B.

[0107] Alternatively, a group III nitride compound semiconductor havingapproximately the same composition as that of the semiconductor crystallayer C and functions as the cap layer C′ may be grown through crystalgrowth at a growth temperature lower than the crystal growth temperatureT_(C) of the semiconductor crystal layer C. By applying that process,the cap layer C′ can be rather easily vanished around the crystal growthtemperature T_(C) of the semiconductor crystal layer C. That makes itcomparatively easier to design the cap layer C′ to be vanished bythermal decomposition with no excess or deficiency. Further, even whenthe cap layer C′ cannot be removed and a portion of it remains on theprotection layer B, because the cap layer C′ has the same composition asthat of the semiconductor crystal layer C, it is possible or easier tostart (or carry on) growing the semiconductor crystal layer C havingexcellent quality through crystal growth.

[0108] In the above embodiments, alternatively other crystal growthmethod such as MOVPE can be employed in place of halide vapor phasegrowth (HVPE). That is, only one crystal growth process may be carriedout without employing the second crystal growth process.

[0109] While the present invention has been described with reference tothe above embodiments as the most practical and optimum ones, thepresent invention is not limited thereto, but may be modified asappropriate without departing from the spirit of the invention.

1. A method for producing a semiconductor crystal having excellentquality and independent from a base substrate on which a semiconductorcrystal comprising a group III nitride compound semiconductor is grown,comprising: single crystal growth process c in which an objectivesemiconductor crystal layer C comprising a group III nitride compoundsemiconductor is deposited; and separation layer removing process α inwhich a separation layer A which comprises a group III nitride compoundsemiconductor and is formed between a protection layer B comprising agroup III nitride compound semiconductor and having crystal growth fronttoward said semiconductor crystal layer C and said base substrate isevaporated through thermal decomposition, wherein said separation layerremoving process α is carried out before the single crystal growthprocess c.
 2. A method for producing a semiconductor crystal accordingto claim 1, wherein said group III nitride compound semiconductor ismade of Al_(1-x-y)Ga_(y)In_(x)N (0≦x≦1, 0≦y≦1, 0≦1-x-y≦1).
 3. A methodfor producing a semiconductor crystal according to claim 1, wherein saidbase substrate comprises at least one selected from the group consistingof sapphire, spinel, manganese oxide, lithium gallium oxide (LiGaO₂),molybdenum sulfide (MoS), silicon (Si), carbon silicide (SiC), AlN,GaAs, InP, GaP, MgO, ZnO, and MgAl₂O₄.
 4. A method for producing asemiconductor crystal according to claim 1, wherein relationship amongsaid heat treatment temperature T_(X) in said separation layer removingprocess α, said crystal growth temperature T_(C) of said semiconductorcrystal layer C, and said crystal growth temperature T_(B) of saidprotection layer B is T_(B)<T_(X)≦T_(C).
 5. A method for producing asemiconductor crystal according to claim 1, wherein said protectionlayer B is formed to have indium (In) composition ratio smaller thanindium (In) composition ratio of said separation layer A.
 6. A methodfor producing a semiconductor crystal according to claim 1, wherein saidsemiconductor crystal layer C is made of gallium nitride (GaN) at atemperature of 900° C. to 1100° C. through crystal growth.
 7. A methodfor producing semiconductor crystal according to claim 1, wherein saidprotection layer B is made of Ga_(1-x)In_(x); N (0.0≦x≦0.8) throughcrystal growth.
 8. A method for producing semiconductor crystalaccording to claim 1, wherein said separation layer A is formed throughcrystal growth by using Ga_(1-x)In_(x)N (0.05≦x≦1.0).
 9. A method forproducing semiconductor crystal according to claim 1, wherein saidtemperature T_(X) of heat treatment carried out in said separation layerremoving process α is in a range from 700° C. to 1100° C.
 10. A methodfor producing semiconductor crystal according to claim 1, furthercomprising: a protection layer removing process β in which saidprotection layer B is evaporated by heat treatment is carried out aftersaid single crystal growth process c.
 11. A method for producingsemiconductor crystal according to claim 10, wherein heat treatmenttemperature in the protection layer removing process β is in a rangefrom 900° C. to 1200° C.
 12. A method for producing semiconductorcrystal according to claim 1, further comprising: a cap layer depositingprocess in which a cap layer, which protects the protection layer B fromthermal decomposition in the separation layer removing process α, isdeposited on the protection layer B before the separation layer removingprocess α.
 13. A method for producing semiconductor crystal according toclaim 12, wherein said cap layer comprises a group III nitride compoundsemiconductor having approximately the same composition as that of saidsemiconductor crystal layer C and is grown through crystal growth at agrowth temperature lower than said crystal growth temperature T_(C) ofsaid semiconductor crystal layer C.
 14. A method for producingsemiconductor crystal according to claim 12, wherein a gallium nitride(GaN) is grown through crystal growth as said cap layer at a growthtemperature from 800° C. to 1000° C.
 15. A method for producingsemiconductor crystal according to claim 1, wherein said crystal growthof all or at least the latter part of said single crystal growth processc is carried out by a second crystal growth having a growth rate fasterthan a growth rate of said crystal growth which had been carried out bythat time.
 16. A method for producing semiconductor crystal according toclaim 1, wherein said crystal growth temperature T_(A) of saidseparation layer A and said crystal growth temperature T_(B) of saidprotection layer B are arranged from 400° C. to 900° C.
 17. A method forproducing semiconductor crystal according to claim 1, whereinrelationship between crystal growth temperature T_(A) of the separationlayer A and crystal growth temperature T_(B) of the separation layer Bis T_(A)≦T_(B).